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Binocular Universe: For the Birds

Binocular Universe:

For the Birds

This month’s column is for the
birds. Literally, as we visit a nebulous aviary along the gentle stream
of the southern Milky Way populated by a swan and an eagle. We paid a call
here last summer, when we focused on M24, the Small Sagittarius Star
Cloud. This year, we return to explore a few more showstoppers in and
around the area.

North of M24, along the hazy
lane of our galaxy, lay three more members of Charles Messier’s catalog: M16,
M17, and M18. All three fit into the field of my 10x50 binoculars, making them
fun to compare. Let's take a look at each. We will begin with the southernmost
of the trio, M18, and work our way northward.

Messier himself was first to
bump into his catalog’s 18th member. He spotted it on June
3, 1764, while he was surveying M17 and M18. His notes recall:
“A cluster of small stars, a little below above nebula, No. 17, surrounded by
slight nebulosity, this cluster is less obvious than the preceding, No. 16;
with an ordinary telescope of 3.5-foot [focal length], this cluster appears
like a nebula; but with a good telescope one only sees small stars.”

Of the three Messier targets
profiled this month, I have to admit that M18 is the least impressive. But
thanks in large part to its surroundings and the company it keeps, it still
presents a pleasant view through binoculars.

Some 40 stars populate this
open cluster, all residing a little more than 4,200 light years away. The
brightest of the bunch are set in a triangular pattern that may just be
glimpsed in 10x50s, but only under dark conditions. The rest blend into a small
knot of hazy starlight.

Studies of the stars’ spectra
given an overall age of the cluster as about 50 million years. That’s
relatively young as star clusters ago. But, as we will see in a moment,
M18 is considerably older than our next two targets.

M17 is one
of my favorite summertime targets and is an easy catch through nearly all
binoculars. The Swiss observer Phillippe Loys de Chéseaux was first to
lay eyes on it in 1746 (N.B. some references say 1745), when he described: “It
has perfectly the form of a ray, or of the tail of a comet, of 7' length and 2'
width; its sides are exactly parallel and rather well terminated, as are its
two ends. Its middle is whiter than the borders.”

Interestingly, Messier did not
know of de Chéseaux’s discovery when he first spied it that same evening of
June 3rd in 1764. So, even though he was technically beaten to the punch,
we still must give Messier kudos for a great find. He saw it as: “A train of
light without stars, of 5 or 6 minutes in extent, in the shape of a spindle,
and a little like that in Andromeda's belt [M31].”

M17 is located just to the
south of a yellowish 5th-magnitude star and less than half a binocular field
north of M18. If you're viewing through 50-mm and smaller binoculars,
look for a straight "bar" of grayish light oriented more or less
southeast-northwest. Larger binoculars add a faint hook-shaped appendage
curving off the western end of the bar.

It's this hook-and-bar shape
that has given rise to one of M17's nicknames, the Swan Nebula. Through
telescopes, the bar represents the swan's floating body, while the hook turns
into its long, graceful neck and head. My 16x70 binoculars portray the swan as
floating upside down, which I find worrisome. Telescopes, of course, invert the
view to show that the swan is alive and well.

Like the Orion Nebula, M42, M17
is an active star-forming region. A young star cluster lies hidden amid
and behind the clouds of M17, though we are hard-pressed to see them directly.
None of the more than 8,000 to 10,000 cluster members are more than a
million years old, mere babes in the heavens. They are separated from us
by 5,900 light years.

M17 is better known by a second
nickname, the Omega Nebula. That analogy stems from an observation made by John
Herschel in 1823, when he wrote “A large extended nebula; its form is that of a
Greek Omega…” You would likely need the aperture of a telescope, however,
to see the full horseshoe shape that led to Herschel’s comment.

Finally, we cross into Serpens
to find the open cluster M16. Like M17, M16 was first eyed by de
Chéseaux in 1746 (or maybe 1745, depending on who you ask) and independently
found by Messier on the busy night of June 3, 1764. Messier described a
“Cluster of small stars, mingled with a faint glow.” True, the stars of
M16 are mingled with the faint glow of ionized hydrogen – the so-called Eagle
Nebula – but Messier did not see that. Instead, his description
points to stars too faint for his telescope to resolve. The nebula that
today we call the Eagle Nebula was not discovered until 1895 by Edward
Barnard. It was subsequently cataloged separately as IC 4703.

The view through binoculars
matches Messier’s words very closely. With my 10x50s, I can count about a
dozen stars against the unresolved glow of fainter suns. With my 25x100
giants and nebulae filters held between eyepieces and eyes, I can catch hints
of the nebula itself, but only on the best nights.

Studies show that M16 is one of
the youngest star clusters known, perhaps no more than five million years old,
affording astronomers an excellent laboratory for study. Its hottest
stars, spectral type O, are seven times hotter than our Sun. M16 is
estimated to be 5,600 light years away.

M16 (actually, IC 4703)
captured the public’s imagination when the Hubble Space Telescope’s “Pillars of
Creation” photograph was released in 1995. This has to be one of the most
dramatic views of the Universe captured in recent years. There, created
by a process known as photo-evaporation, columns composed of cooled hydrogen
and dust appear to be reaching into the surrounding cloud, as if to grasp at
the stars spawned within. The Space Telescope Science Institute notes
that as the pillars themselves are slowly eroded away, small globules of even
denser gas buried within the pillars are uncovered, dubbed "EGGs," an
acronym for Evaporating Gaseous Globules. Eventually, most these
embryonic suns will evolve into mature stars.

As you can see from the chart
above, there are plenty of other targets in the immediate area. If you
have the chance to view under dark, transparent skies, try your luck with a
couple of the dark nebulae toward the eastern half of the chart. Here are some
details.

Questions, comments, suggestions? Let’s talk! Post them in this
blog’s discussion forum. Until next month, remember that two eyes are better
than one.

About
the Author:

Phil Harrington is a
contributing editor to Astronomy magazine and author of 9 books on
astronomy. Visit his web site at www.philharrington.net

Phil
Harrington's Binocular Universe is copyright 2014 by Philip S.
Harrington. All rights reserved. No reproduction, in whole or in
part, beyond single copies for use by an individual, is permitted without
written permission of the copyright holder.